The last major version of IRIX was IRIX 6.5 which was released in May 1998. New minor versions of IRIX 6.5 were released every quarter until 2005; since then there have been four further minor releases. Through version 6.5.22, there were two branches of each release: a maintenance release (identified by an m suffix to the version number) that included only fixes to the original IRIX 6.5 code, and a feature release (with an f suffix) that included improvements and enhancements. An overlay upgrade from 6.5.x to the 6.5.22 maintenance release is available as a free download, whereas versions 6.5.23 and higher require an active Silicon Graphics support contract, despite only running on Silicon Graphics hardware.

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The IRIX name was first used around the time of release 3.0 of the operating system for SGI's IRIS 4D series of workstations and servers, in 1988. Previous releases were identified only by the release number prefixed by "4D1-", e.g. "4D1-2.2". The 4D1- prefix continued to be used in official documentation to prefix IRIX release numbers.

In 1994, IRIX 6.0 added support for the 64-bit MIPS R8000 processor, but was otherwise similar to IRIX 5.2. Later 6.x releases supported other members of the MIPS processor family in 64-bit mode. IRIX 6.3 was released for the SGI O2 workstation only.[4] IRIX 6.4 improved multiprocessor scalability for the Octane, Origin 2000, and Onyx2 systems. The Origin 2000 and Onyx2 IRIX 6.4 was marketed as "Cellular IRIX", although it only incorporated some features from the original Cellular IRIX distributed operating system project.[5] IRIX development stabilized with IRIX 6.5, released in 1998. The last version of IRIX was 6.5.30, released in August 2006.

A 2001 Computerworld review found IRIX in a "critical" state. SGI had been moving its efforts to Linux and Windows,[a] but MIPS/IRIX customers had forced it to continue to support that platform through 2006.[6] On 6 September of that year, an SGI press release heralded the end of the MIPS/IRIX product line .[7] Production ended on 29 December 2006 with last deliveries in March 2007, except by special arrangement. Support for these products ended in December 2013 and they will receive no further updates.[8] All current SGI systems are X86-64 based and shipped with Red Hat Enterprise Linux or SUSE Linux Enterprise Server.

IRIX used the MIPSPro 7.4 Compiler for both its front end and back end. The compiler was designed to support parallel POSIX programming in C/C++, Fortran 77/90, and Ada. The Workshop GUI IDE was used for development. Other tools include Speedshop for performance tuning, and Performance Co-Pilot.[10]